The present invention relates to an improvement in hydraulic control valves and more particularly to a control valve used for a hydraulic clutch and hydraulic brake operation like a power take-off shaft in agricultural tractors.
Most conventional agricultural tractors incorporate a power take-off shaft that is used to drive accessory equipment such as trailing implements, which must be power driven. In recent years, the power take-off shafts have been connected to a power source, such as an engine, through a hydraulically actuated clutch. The power take-off shaft also incorporates a hydraulically actuated brake which prevents rotation of the shaft when it is not desired.
Recently, it has been proposed to operate the clutch and brake associated with a power take-off shaft with a single valve to simultaneously engage the clutch and disengage the brake and vice versa. It has also been found desirable to use a detent mechanism for holding the control valve in one position or another. Historically, detent mechanisms have been of the mechanical type, such as a spring detent in which the spring force can be manually overcome. However, spring detents have the disadvantage in that the valve remains in a certain position until a manual force is applied by the operator.
For safety reasons, it is desirable to have the control valve automatically move to a clutch disengaged position when the hydraulic system no longer supplies fluid, such as when the engine is stopped. This prevents accidental starting of the engine when with the power take-off shaft engaged which may result in injury to a person adjacent the vehicle.
U.S. Pat. No. 3,722,542 discloses an automatic pressure control valve for controlling the flow of fluid under pressure to the clutch and brake of a power take-off shaft. The control valve disclosed in the above-mentioned patent incorporates a hydraulic lock for holding the valve in an actuated position and the valve is arranged so that it automatically disengages the clutch and engages the brake when power is lost. Thus, the clutch is always disengaged when the system no longer supplies pressurized fluid. However, one of the problems encountered with the control valve of the above-mentioned type is the fact that, when the operator wants to manually disengage the power take-off shaft, it is necessary to exert a large force on the control lever to force the trapped fluid from the end of the valve spool through a restricted orifice in order for the operator to move the spool to the neutral position. This may be hazardous condition, particularly when the operator wishes to rapidly disengage the power take-off shaft. Another problem inherent in the construction of the control valve is the complicated arrangement required for gradually increasing the pressure of the fluid as the clutch is being actuated.
It has also been proposed to utilize hydraulically actuated detent mechanism. U.S. Pat. No. 3,869,107 discloses a detent system wherein the detents are moved through the use of a separate pilot pressure source. The system disclosed in this patent not only requires a separate fluid source for moving the detent plungers but also requires an additional pilot valve and a complicated plumbing system which increases the overall cost of the system.